1. Field of the Invention
This invention relates to an image reading apparatus for reading, by means of optical scanning, a document placed on a document platen, and more particularly to an image reading apparatus which is capable of focussing a lens during the image capture by detecting the leaf face height.
2. Description of the Related Art
The image reading apparatus, which is designed to capture an image of a document so placed as to direct the leaf face upward, has been heretofore known to adopt the apparatus which scans the edge of the terminal end of a document, detects the difference in brightness between the back end and the background, thereby recognizes the shape of the relevant part of the document and detects the leaf face height (Japanese Published Unexamined Patent Application (A) 07-079324). According to this technique, a detector as an image sensor scans and reads the leaf face placed on a document platen and the terminal end of the document reflected by a mirror disposed aslant behind the rear side of the document platen at the same time. The apparatus, therefore, is enabled to determine the leaf face height by counting the number of picture elements of the image sensor corresponding to the edge of the terminal end of the document which has been read out. It is enabled to bring all the points of the leaf face into focus and obtain a perfect in focus image of the document.
Further, there is a known image reading apparatus (JP-A-60-254869), which is provided on the rear side of a document platen with a distance measuring plate as a stopper for positioning of a document and is so designed to press against the distance measuring plate the top end of a document (on the rear side) so placed as to direct the leaf face upwardly, capture the edge of the top end of the document, detect the difference in brightness between the leaf face and the distance measuring plate disposed on the background, and correct the read image, based on the difference of brightness. In this apparatus, the surface of the distance measuring plate forms a focus detecting surface with a reflectance different from that of the document platen and that of the document surface. When a given document is thick, as in the case of a book or a document, and the thickness varies in the lateral direction, part of the focus detecting surface is covered on account of the thickness of the document. As a consequence, there is a change in the length in the main-scanning direction (recessing direction) of the focus detecting surface which is discerned by an image sensor such as a linear CCD image sensor, for example. By discerning this length, the apparatus can detect the variation in the thickness of the document and focus a lens on the image.
The image reading apparatus as disclosed in Japanese Patent Application No. 05-169720, which is so designed to focus a lens on an image based on the height of the document detected by scanning the terminal end of the document, has a possibility that binding rings are mistaken for the leaf face when a given document having opposite faces bound with the binding rings is placed on the document platen. It is at a disadvantage when exposing the focus to deflection and inducing the phenomenon of blurring in the produced image.
Incidentally, such binders used for binding pages of the document produce shadows at the center of the leaf face. The shadows, therefore, create the problem of obscuring the image being read out.